librbd: expand API to include namespaces and image ids

librbd: expand API to include namespaces and image ids

The get parent and list children methods now return a common
struct that includes the full pool details (including the
optional namespace). Additionally, a new image list method has
been added which includes both the name and id (of v2 images).

Fixes: http://tracker.ceph.com/issues/36650
Signed-off-by: Jason Dillaman <dillaman@redhat.com>